1. Field of the Invention
This invention relates to a method for measuring electrolyte and an apparatus for measuring electrolyte for measuring the concentration (activity) of particular predetermined ions in an aqueous solution such as liquor, city water and body fluids (e.g., blood, blood plasma, blood serum, lymph, cerebrospinal fluid and urine) in potentiometric way by use of a device for measuring ionic activity.
2. Description of the Prior Art
There is disclosed, for instance in U.S. Pat. Nos. 4,053,381 and 4,437,970, a device for measuring ionic activity which makes it feasible to determine the activity of particular predetermined ions in sample liquid from a droplet of the sample liquid deposited thereon by spotting.
The device for measuring ionic activity comprises a pair of ion-selective electrodes each of which generates a potential corresponding to the activity of particular predetermined ions, a pair of spotting holes through which a sample liquid (a liquid wherein the activity of particular predetermined ions is unknown) and a reference liquid (a liquid wherein the activity of the particular predetermined ions is known) are respectively supplied to the ion-selective electrodes and a porous bridge for ionic contact which communicates the spotting holes with each other. In the device for measuring ionic activity, the activity of the particular predetermined ions in the sample liquid is determined in the following manner. That is, when the sample liquid is supplied through one of the spotting holes and the reference liquid is supplied through the other spotting hole, the liquids are applied to the respective ion-selective electrodes and at the same time diffuse in the porous bridge by capillarity to come into contact with each other at a liquid phase boundary. This contact between the sample liquid and the reference liquid establishes an electrical conduction between the electrodes, and a potential difference is generated between the electrodes according to the difference in the activity of the particular predetermined ions between the sample liquid and the reference liquid. By measuring the potential difference, the activity of the particular predetermined ions in the sample liquid can be determined.
It has been believed that, in order to determine the activity of the particular predetermined ions in the sample liquid with high accuracy, the sample liquid and the reference liquid must form a phase boundary at the center of the porous bridge. Accordingly, the sample liquid and the reference liquid are conventionally spotted to the spotting holes simultaneously with each other. As a device for facilitating such simultaneous spotting, there has been known a dual pipette device disclosed, for instance, in U.S. Pat. No. 4,801,434.
Though the dual pipette device is for manually spotting the sample liquid and the reference liquid, it is preferred that spotting of the liquids be automated in order to measure a number of sample liquids at high speed. In U.S. Pat. No. 4,347,750, there is disclosed a device for automating spotting of the sample liquid and the reference liquid. In the device, a reference liquid spotting tip which sucks a reference liquid in a reference liquid sucking position and spots the reference liquid to one of the spotting holes in the device for measuring ionic activity in a reference liquid spotting position is arranged to be movable between the reference liquid sucking position and the reference liquid spotting position independently from a sample liquid spotting tip which sucks a sample liquid in a sample liquid sucking position and spots the sample liquid to the other spotting hole in a sample liquid spotting position. In the reference liquid spotting position, the reference liquid spotting tip is held obliquely though the sample liquid spotting tip is held vertically in the sample liquid spotting position. This is for the following reason. That is, in order to simultaneously spot the sample liquid and the reference liquid, the spotting tips must be simultaneously positioned above the corresponding spotting holes. However since the space between the spotting holes is generally narrow (e.g., 8 mm), the parts for holding the spotting tips interfere with each other when the spotting tips are positioned above the spotting holes with both the spotting tips held vertically. By obliquely holding the reference liquid spotting tip, interference between the parts for holding the spotting tips is prevented and the sample liquid and the reference liquid can be simultaneously spotted.
However when the reference liquid spotting tip is held obliquely, it is difficult to accurately spot the reference liquid to the spotting hole. Further the mechanism for obliquely holding the reference liquid spotting tip is complicated in structure, which increases the cost of the overall apparatus. As a result, the cost of measurement of electrolytes by use of the device for measuring ionic activity increases.